Cellulosic pellicles and process for preparing same



Patented Aug. 29, 1939 UNITED s'rarss PATENT orr cs I CELLULOSIO PELLICLE'S AND PROCESS FOR PREPARING SAME ration of Delaware No Drawing. Application January 4, 1936, Serial No. 57,632

20 Claims.

This invention relates to flexible, cellulosic pellicles and particularly to such pellicles which have been rendered resistant to change in dimensions caused by variation in humidity con- 5 ditions.

Cellulosic pellicles which are obtained by agulation or precipitation from aqueous or alkaline aqueous dispersions of cellulose or cellulose derivatives, as for example, pellicles of regen- 10 erated cellulose, glycol cellulose, cellulose glycollic acid, lowly esterified or etherified cellulose such as lowly etherifled methyl and ethy cellulose and lowly esterified cellulose acetate, are particularly r useful as wrapping materials. It is essential, however, that these pellicles be flexible and further that this flexibility be maintained. Cellulosic pellicles of this character seem to depend on the presence of moisture to impart flexibility and the softeners which have been used heretofore have 20 been selected because of their hygroscopic nature.

It has been considered necessary to have the softener assist, by virtue of its hygroscopicity, in the retention of a suitable amount of moisture although the softener has possessed, also, a certainv 25 degree of lubricating or plasticizing action so that the pellicle does not become too brittle when practically all of the moisture has been removed. In the absence of a softener other than water, these pell cles become so brittle when dried as to be substantially useless as wrapping tissues. In the presence of the hygroscopic softeners of the prior art, the loss of moisture is retarded and generally speaking, the flexibility is mainta ned by the moisture present.

Regenerated cellulose pellicles (for convenience the invention will be described in terms of this species) are quite sensitive to changes in moisture content, not only as regards flexibility, but also as regards dimensions. Increase in moisture content causes a swelling of the cellulosic structure, while decrease in moisture content causes a shrinking apparently due in part to collapse of the cellulose micelles and in part to the loss of water molecules from between the micelles.

Regenerated cellulose wrapping tissues are subject, therefore, to two major defects wh ch develop simultaneously and which are primarily dependent on the softener, namely embrittlement and deformation. Previous attempts. to control or prevent'embrittlement, however, have not been successful in controlling the deformation for, as mentioned above, the softeners selectedhave been highly hygroscopic and have really resulted in greater sensitivity to moisture conditions, thereby increasing the expansion and contraction of wrapping tissues or similar pellicular structures.

Thus it is that great care is required in wrapping boxes in regenerated cellulose pellicles to provide for these conditions. For example, if a cereal box is wrapped in regenerated cellulose sheeting (prepared in accordance with the prior commercial methods and softened with glycerin which is the usual hygroscopic softener) and set aside for storage, it may be subjected to a variety of humidity conditions before it reaches its ultimate consumer. During this time, if the humidity is high, the regenerated cellulose may expand until the wrapper becomes loose around the box and in some cases even baggy and wrinkled. On the other hand, if the humidity is low, loss of moisture from the regenerated cellulose will cause the wrapper to contract and this may cause buckling of the box walls, or if the box is sufficiently rigid, the wrapper itself may burst. Thus, under such a variety of humidity conditions, packages wrapped in regenerated cellulose sheeting may. have an unsightly and undesirable appearance as the result of the deformation of the regenerated cellulose. Obviously, this defeats the very im- To overcome the troublesome deformation;

wrapping machinery has been designed to allow for a certain slack or looseness in the wrap.'- This obviously will eliminate the effects of contraction-: but cannot help the expansion effects; indeed; t makes them worse.

regenerated cellulose pellicles does not-eliminate the trouble because the application ofa mois-* tureprooflng coating retards but does not prevent the deformation, which latter is in the ultimate not appreciably affected. These practical means do nothing more than attempt to make the best 40 of the situation and make use of regenerated cellulose sheeting as it is available. No attempt is made to change the inherent properties of the cellulosic material and so provide a wrapping which will have improved properties and characteristics.

It has been found in the manufacture of re: generated cellulose pellicles that the degree of deformation is, at least in part, due to the orientation of the cellulosic micelles. Thus, if asheet of regenerated cellulose is made in such a way as to provide substantially uniform tension in all directions, the degree of deformation will be substantially the same in all directions. Practically, however, regenerated cellulose pellicles are made The use of moistureprobfe'd i right angles to the machine direction, i. e., the

transverse direction. However, in any given process the conditions are such that the degree of deformation in the machine and transverse directions usually bear a sufliciently constant relationship so that for test purposes it is usually sufiicient to determine the degree of deformation in one direction only, usually the machine direction.

For the purposes of this specification the degree of deformation, which may be called simply deformation", is the per cent change in length of a cellulosic pellicle as measured in the machine direction in accordance with the following procedure: Strips of material are allowed to come to equilibrium with an atmosphere of substantially 95% relative'humidity at a temperature of 35 C. and their length accurately measured. The strips are then allowed to reach equilibrium in an atmosphere of substantially 0% relative humidity at the same temperature and their length again accurately measured. The difference in length divided by the length as originally measured multiplied by 100 gives the per cent deformation over the given humidity range at 35 C. and figures so obtained are conveniently called the deformation. Thus, a sample strip having an original length of inches and a contraction in length of 0.42 inch would be said to have a deformation of 4.2.

It is the object of this invention to provide means for reducing the deformation of cellulosic pellicles. It is also an object of the invention to effect simultaneous softening and reduction of deformation. It is a further object to provide a method whereby the aforementioned objects may be accomplished in an economically feasible manner and without entailing essential modification of apparatus customarily used in the manufacture of such pellicles.

More specifically, it is the object of the invention to provide means for the production of cellulosic pellicles which will show less deformation than the cellulosic pellicles presently commercially available.

Specifically, it is the object of the invention to produce a pellicle of regenerated cellulose suitable for use as a wrapping tissue which will show a degree of deformation of not in excess of 3.0 when tested in accordance with the method described above. This object also includes the production of a regenerated cellulose pellicle which is combined with a softening material and which shows a deformation not in excess of 3.0.

The above and other objects of the invention may be accomplished by impregnating into the cellulosic pellicle, preferably while the latter is in a wet or gel state, an appreciably water-soluble, high-boiling (i. e., relatively non-volatile at ordinary temperatures and pressures), limitedly hygroscopic material which is stable, essentially colorless and odorless, preferably non-toxic and which is preferably also a softening agent for the cellulosic material. The extent of the reduction in deformation which may be obtained depends on the particular impregnant used and to a certain extent on the amount employed.

The term limitedly hygroscopic" is intended to include substances which will absorb 1-80% of their weight of water when exposed alone in a thin layer to an atmosphere of substantially 95% relative humidity at a temperature of 25 C. over a period of 120 hours. The test for hygroscopicity is carried out as follows: A small sample of about 0.5-2.0 grams of thoroughly dried material is spread evenly over the bottom of a weighing bottle (conveniently about 2% in diameter and 1 /1 deep) and the weight of the sample accurately measured. The open weigh-. ing bottle is then placed in a chamber in which an atmosphere of substantially 95% relative humidity and a temperature of 25 C. is maintained. The humid atmosphere may be maintained conveniently by means of a sulfuric acid solution (9 parts of water to 1 part sulfuric acid) contained within the chamber. The sample, after 120 hours exposure, during which time the material is occasionally agitated as by gently tipping the container to cause the material to flow over the bottom of the weighing bottle, is reweighed accurately and the percentage increase in weight based on the original weight of the sample reprwents the hygroscopicity of the material. Thus, if 1 gram of a substance absorbs 0.25 gram of water under the conditions described, it will be said to have a hygroscopicity The test for hygroscopicity as outlined above is satisfactory for determining whether or not. a given substance is suitable for the purposes of the invention. There may be other tests of equal utility for determining hygroscopicity which would give different numerical values. Obviously, such methods should be calibrated against the method outlined if they are chosen for use. Based on the method described, those substances suitable for use in the practice of the invention will have a hygroscopicity of 1 to 80.

Generally speaking. the test of water-solubility is applied first to determine the suitability of substances for use in the practice of the invention. If the substance is at least 1% and not more than 40% soluble in water at 20 0. its utility is indicated. In the case where the solubility is more than 40%, the hygroscopicity should be determined and in the event that this value should be above 80, the substance will be useless for the purposes of the invention.

The third physical criterion relates to volatility. The substances must be high-boiling, that is, substantially non-volatile at ordinary temperatures and pressures, and should be preferably though not necessarily liquids. Obviously, if substances of appreciable volatility were to be used, they would eventually disappear from the cellulosic pellicles rendering them thereby brittle,

. fragile and unsuitable for use as wrapping tissues or other applications. It has been found that substances of appropriate water-solubility and hygroscopicity which have a boiling point of 135 C. or higher, at a pressure of 12 mm. of

mercury and preferably of 150 C. or higher (at of a simple watersolution. In the case where it is desired to use'a substance whichis less than 4% soluble, it maybe convenientto increase the concentration by adding a wate'fiinisiblorganic solvent such as methanol, ethanol, acetone or the like although in no'ca'se'shouldmore'than 25% by volume of such solvent in the watersolvent mixture be necessary. Substances which require more organic solvent are unsuited for the part of the invention to provide also a flexible pellicle. This can be done frequently by proper 7 choice of the impregnating agent so that it will be at one and the same time a softener and a deformation reducing agent. For simplicity, such an-agent may be termed a non-deforming softener. Obviously, other properties may be combined with these in certain agents, which will have particular advantage.

The non-deforming softeners suitable for use in accordance with the present invention are esters containing an ether group, said esters being esters of carboxylic acids. Non-deforming softeners which are especially suitable for use comprise the ether-esters derived from etheralcohols. These may be synthesised by the ester ification of monoor polycarboxylic acids with alcohols which contain ether groups in their molecular structure. As the acid portion of the ester, propionic, acetic, isobutyric, maleic, adipic, succinic, acetyl or propionyl malic acid or other aliphatic acids may be used. Likewise, acids containing cyclic or aromatic groups are useful although generally their esters are not sufficiently water-soluble. The alcohol portion of the ester may be provided by:

l. Monohydric alcohols containing one or more ether groups as for example the mono-alkyl ethers of ethylene, propylene, diethylene or polyethylene glycols.

2. Dihydric alcohols containing one or more ether groups such as diethylene glycol, triethylene glycol, 2-hydr0xy-cyclohexyl ether of ethylene glycol, or the like.

3. Trior polyhydric alcohols containingone or more ether groups such as di-betahydroxyethyl ether of glycerol, etc.

Although the preparation of these ether-esters does not constitute a part of the instant invention, it should be understood that they may be made in any desired way provided that the ester product is non-resinous. Thus a monohydric alcohol and a monoor polybasic acid, or a poly hydric alcohol with a monobasic acid will pro duce esters which are useful. If, however, a polybasic acid and a polyhydric alcohol are esterified, care must be taken to avoid resin formationif the ester is to be used in the practice of the invention, for those compounds which are useful, are simple organic ether-esters and are non-resinous in character. It is understood also that only those ether esters which satisfy the above described requirements for hygroscopicity, water-solubility and non-volatility will be of value in practicing the invention.

-As illustrative. of specific materials which are useful inthe practice of the invention, a number of compounds are listed in the following table together with data as to water-solubility and deformation, the. latter figures. being obtained when regeneratedcellulo'se pellicles were impregnated as will be described in more detail below.

Table a- Approximatewater Deformasolubility tion at 20 C.

Percent (11) Bis-ethoxyethylsucclnate 8 2.2 (b) Bls-rnethoxyethyl adipatc. 1i 2. 3 (c) Propionatc of bis-methoxyethyl malate. 7 2.1 .(d) Acetate of b s-methoxyethyl malatc. 24 2.7 (e) Acetate of bls-ethoxyethyl malate 2 2. 5 (f) Bls-ethoxyethyl maleate 5 2. 6

The compounds listed in the table are representative and it is to be noted that simple organic chemical compounds are effective for the purposes of the invention. All of the substances listed satisfy the criteria for operability previously discussed. I

The impregnating agents of the invention may be introduced into cellulosic pellicles in the same way that glycerol is usually introduced in the manufacture of present-day commercial regenerated cellulose sheeting. Thus, a sheet of gel regenerated cellulose which has been purified and washed is impregnated with an aqueous glycerol solution, the excess of such solution removed by suitable means and the sheet dried; The commercial operation is continuous and the time of immersion in the glycerol bath varies with the speed of the machine although about 20 seconds immersion is probably normal. The concentration of the glycerol in the bath is adjusted to leave a predetermined amount of glycerol in the sheet after the drying operation, which latter removes all but about 6-8% water, based on the weight of the cellulose. Generally, the amount of glycerolin the final product is of the order of 15% and it has been found that a bath containing about 4- 6% glycerol will give the desired results.

Accordingly, in the practice of the present invention the gel pellicle is impregnated with a bath which may contain conveniently about 4-6% of the non-deforming agent. After removal of excess bathfas by squeeze rolls and the removal of excess water by drying, the final product will contain a suitable amount of non-deforming agent. Obviously, the amount of such agent in the final product will be adjusted to suit the desired properties expected in the final sheet so that the bath concentration may be more or less than 4% as occasion demands. It has been found however that the effectiveness of most non-deforming agents appears to approach a limiting value so that excessive amounts do not produce 'sumcient improvement to justify their use. Usually a bath concentration of about 4% will be found satisfactory for a realization of good non-deforming properties and at the same time economy of oper-- ation. Where the sheet is to be used at low temperatures and low humidities, a 6% solution may be found advantageous.

As a specific example, a gel sheet ofregenerated cellulose may be impregnated with a 4% aqueous solution of bis-ethoxyethyl succinate. The resultant sheet, after drying shows a deformation of about 2.2, whereasa similar sheet impregnated with glycerol shows a deformation of 4.04.4. Both sheets contain approximately 14% of the impregnating agent. In this case the product is flexible, durable, transparent and shows a reduction in deformation as compared with the glycerin softened sheet amounting to approximately 50%.

In the above table a number of compounds has been indicated, each of which is capable of use in the production of a regenerated cellulose pellicle which will show a deformation of not more than 3.0. It is to be understood that the compounds mentioned are illustrative. The list given is by no means exhaustive and this disclosure is intended to embrace all organic compounds or mixtures of the class defined which have properties as previously set forth. These compounds may be used alone or in combination with each other.

The hygroscopicity of the compound chosen for use will determine, in large measure, the amount of reduction in deformation which will be observed. Thus, compounds having a hygroscopicity of from 60 to 80 will show in most cases a deformation of the order of 2.7 to 3.0, while if the hygroscopicity is from 25 to 60, a deformation of the order of 2.3 to 3.0 will usually be obtained, and if the hygroscopicity is from 1 to 25 a deformation of about 2.3 or less (usually of the order of 2.0 to 2.3) will be achieved in most instances. Thus, it can be seen that any variation in properties which may be desired in the finished product can be secured by proper selection of a non-deforming agent or non-deforming softener in accordance with the principles set forth herein.

Thecellulosic pellicles obtainable by means of the present invention are particularly suited to wrapping purposes since the reduced deformation substantially eliminates warping, swelling, wrinkling and breakage. Similarly, the lamination of such'pellicles to materials such as paper or fabric is facilitated since there is less tendency for the laminated product to curl, buckle or wrinkle. when stacks of sheets are stored, there is a lesser tendency for contiguous sheets to stick-a com- 'mon experience with glycerol-softened sheets. Generally, the durability, particularly at low temperatures of C. or less, is improved. The process described is especially advantageous from an economic viewpoint inasmuch as it offers a method of producing this new type of cellulosic pellicle without alteration of present manufacturing equipment.

While it is preferred that the cellulosic pellicle be impregnated with a solution of the non-deforming agent while the pellicle is in the gel state, i. e., before it is dried and while it still contains a large amount of water, it is within the broad scope of the invention to impregnate a film which has been dried and then rewetted.

Although the invention has been described with specific reference to films of regenerated cellulose prepared by the viscose process since it is in this field that the invention is of greatest utility, it will be understood that the broad scope of the invention includes the treatment of other types of cellulose film cast from aqueous or aqueous alkaline solutions. Thus, the non-deforming agents referred to above may be used with advantage in the treatment of films produced from cuprammonium cellulose solutions, from aqueous alkaline solutions of glycol cellulose or cellulose glycollic acid, from aqueous alkaline solutions of lowly etherified cellulose, e. g., lowly etherified methyl cellulose or lowly etherified ethyl cellulose, or from aqueous alkaline solutions of lowly esterified cellulose, e. g., lowly esterified cellulose acetate.

Since the invention is capable of considerable variation and modification, any change from the above specific details and examples which conforms to the spirit of the invention is intended to be included within the scope of the claims.

I claim:

1. Water-sensitive film suitable for use as a wrapping tissue formed from an aqueous alkaline cellulosic solution containing, as a deformationrestricting agent, a softening agent for said film comprising an organic ester containing an ether group having a hygroscopicity of from 1 to 80, a boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C., said ester being present in sufilcient quantity to restrict the deformation of the film to a maximum of 3.0.

2. Water-sensitive film as defined in claim 1 characterized in that said organic ester is an ester of a carboxylic acid and an alcohol containing an ether group.

3. Water-sensitive film as defined in claim 1 characterized in that said organic ester is bisethoxyethyl succinate.

4 Water-sensitive film as defined in claim 1 characterized. in-that said organic ester is bismethoxyethyl adipate.

5. Water-sensitive film as defined in claim 1 characterized in that said organic ester is bismethoxyethyl malate.

6. Regenerated cellulose film suitable for use as a wrapping tissue containing, as a deformation-restricting agent, a softening agent for said film comprising an organic ester containing an ether group having a hygroscopicity of from 1 to 80, a boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C., said ester being present in sumcient-quantity to restrict the deformation of the film to a maximum of 3.0.

7. Regenerated cellulose film as defined in claim 6 characterized in that said organic ester is an ester of a carboxylic acid and an alcohol containing an ether group.

8. Regenerated cellulose film as defined in claim 6 characterized in that said organic ester is bis-ethoxyethyl succinate.

9. Regenerated cellulose film as defined in claim 6 characterized in that said organic ester is bis-methoxyethyl adipate.

10. Regenerated cellulose film as defined in claim 6 characterized in that said organic ester is bis-methoxyethyl malate.

11. A process for reducing the deformation of water-sensitive film formed from an aqueous alkaline cellulosic solution which comprises impregnating said film with an aqueous solution containing a sufficient quantity of a deformationrestricting agent to restrict the deformation of the film to a maximum of 3.0, said deformationrestricting agent being a softener for said film and comprising an ester containing an ether group having a hygroscopicity of from 1 to 80, a boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C.

12. A process for reducing the deformation of water-sensitive film as defined in claim 11 characterized in that said organic ester is an ester of a carboxylic acid and an alcohol containing an ether group.

13. A process for reducing the deformation of water-sensitive film as defined in claim 11' characterized in that said organic ester is bis-ethoxyethyl succinate. V

14. A process for reducing the deformation of water-sensitive film as defined in claim 11 characterized in that said organic ester is bismethoxyethyl adipate.

15, A process for reducing the deformation of water-sensitive film as defined in claim 11 characterized in that said organic ester is hismethoxyethyl malate.

16. A process for reducing the deformation of regenerated cellulose film which comprises impregnating said film with an aqueous solution containing a'sufiicient quantity of a deformation-restricting agent to restrict the deformation of the film to a maximum of 3.0, said deformation-restricting agent being a softener for said film and comprising an organic ester containing an ether group having a hygroscopicity of from 1 to 80, a. boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C.

17. A process for reducing the deformation of regenerated cellulose film which comprises impregnating said film, while in the gel state, with an aqueous solution containing a sufiicient quantity of a deformation-restricting agent to restrict the deformation of the film to a maximum of 3.0, said deformation-restricting agent being a softener for said film and comprising an organic ester containing an ether group having a hygroscopicity of from .1 to 80, a boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C.

18. A process for reducing the deformation of regenerated cellulose film as defined in claim 16 characterized in that said organic ester is bisethoxyethyl succinate.

19. A process for reducing the deformation of regenerated cellulose film as defined in claim 16 characterized in that said organic ester is hismethoxyethyl adipate.

20. A process for reducing the deformation of regenerated cellulose film as defined in claim 16 characterized in that said organic ester is bismethoxyethyl malate. v

HENRY S. ROI'HROCK. 

